Mastech Variacs variable transformers now available

We are pleased to announce the arrival of Mastech Variacs! We currently carry three models of Mastech Variacs, a 5KW or 5 KVA model, a 2KW or 2 KVA Variac, and a 500W or 500VA Variac. We also have two variations of the 5KVA and 2KVA variacs, using 110V AC input or 220V input.

Mastech Variacs are high quality variacs with voltmeter for easy setup, and with very wide range of ouput voltage: from 0V to 250V, to address a wide range of applictions. Please check them out here:

http://www.mastechpowersupplies.com/variac.html

Sean from Mastech Power Supply

How to Set Current Limit for Mastech CC CV Power Supply?

Setting the current limit is a little bit more complicated compared to setting the voltage limit, especially if you have one of the following situations:

1) your load has internal source of EMF, this includes battery, DC motors, RC cars and trains, any electrochemical systems including plating, etching, and electrolysis, etc.

2) your load might be sensitive or damaged by over voltage, e.g., LED

In the first scenario, you need to make sure to set the voltage to slightly above the maximum EMF from your load, say 0.8V to 1.5V above, and set the current to minimum before connecting the load. Once you connect the load, you can slowly increase the current limit by adjusting the current knob to the desired value. It is highly advised to have a diode put in line with your load (connecting to the positive terminal of the power supply) to prevent the current flowing in the wrong direction and causing damage to the power supply.

In the second scenario, if you can find a simple load like a power resistor or a halogen lamp that can handle the current you want to pass, you can set the current similar to the first scenario (Caution: the power resistor or lamp might be very hot if you are passing large current). Once you are done with setting the current limit, turn the voltage down to minimum, disconnect your load of resistor or lamp, then connect you actual load. you can now slowly increase the voltage while leaving the current setting, just remember to always stop before you raise the voltage too high for your load. If your calculation is correct, then you should be able to stop the voltage knob before actually damaging your load.

If you cannot find a power resistor or lamp, you can still set the current limit you want by following the steps below:

Step 1: before connecting your load, set the voltage limit to safely below what's allowed by your load, and set the current to minimum. The power supply should be in "CV" mode.

Step 2: Connect the load; the power supply should be in "CC" mode. Slowly increase the current limit, if you are lucky, you can reach the desired current limit before you need to do anything.

Step 3: If you cannot reach the desired current before the power supply switched over from "CC" to "CV", turn the current slightly down so the power supply is switched back to "CC". When in "CC" mode, you can turn the current knob down a little bit more to have a feel about how much turn corresponds to what current change. Using this gut feel, increase the current knob so that it is half way between your desired current level and the maximum you can get without increasing the voltage setting, now the power supply should be in "CV" mode. You can now increase the voltage knob a little, still making sure that it's below what's safely allowed for your load, and stop when the power supply switches from "CV" to "CC". Now you can see what the actual current is. If the current is higher than your desired value, reduce the current setting to your desired value and you are done. If it's lower than what you desire, repeat Step 3 until you get desired current setting.

It sounds complicated, but when you are actually doing it, it only takes minutes to set it.

Sean from Mastech Power Supply

What is CC and CV Mode of a CC CV Power Supply?

A Mastech Power Supply is a CC CV power supply can be operated in either of the two modes: CC (constant current) and CV (constant voltage) mode. You can tell which mode the power supply is in by looking at the front panel, where either the "CC" or "CV" indicator light is on. Needless to say, if "CV" light is on, it means that the power supply is in constant voltage mode.

If you are a novice user, you may wonder what does it mean? With any Mastech power supply, you can set both the current and voltage using the knobs in the front. By doing so, you are not setting the actual current or voltage, you are merely setting the limits for voltage and current.

If you remember Ohm's Law, you know that at any given load, you can only control either the current or the voltage, the other is determined by Ohm's Law and the load। When the "CV" light is on, it means that the voltage you see on the display (which is always the actual voltage the power supply is putting out) is the same as the voltage you set, while the current you set is higher than the actual current allowed in the particular configuration. This is because with the particular load connected, you have reached the limit set by the voltage knob. Alternatively, when the "CC" light is on, the current on the display (which always shows the actual current flowing through the circuit) is the same as the current limit you set, and the voltage limit you set is higher than the actual voltage displayed on the power supply.

Sean from Mastech Power Supply